MIL-STD-167-1A
5.2.2.2 Balance limits for rigid rotors. When balanced as specified in 5.2.2.1, the maximum allowable residual unbalance is given by the following formula:
U=6GW/N
Where: U is maximum allowable residual unbalance (oz-in)
G is the total balance quality grade (mm/sec) as specified (see 6.2.g(3)) W is weight of the rotor (lbs)
N is the maximum rotor rpm
For rigid rotors which operate below 1000 rpm, the total balance quality grade shall not exceed G=2.5 mm/s. For rigid rotors which operate at 1000 rpm and above, the total balance quality grade shall not exceed G=1.0 mm/s. For rigid rotors which require low noise, a balance quality grade of G=1.0 mm/s can be specified for all speeds (see
6.2.g(3)). For guidance on balance quality grades of rigid rotors, see ANSI S2.19.
In allocating an allowable unbalance (U) between two planes of correction, the allocation ratio must not be more than 2 to 1. The amount allocated to each plane must be proportional to the distance from the other plane to the center of gravity (c.g.) of the rotor divided by the total distance between planes. If the distance between the correction planes were 10 inches, and the c.g. were 4 inches from plane 1, plane 1 would be allowed 60% of U, and plane 2 would be allowed 40%. If the c.g. were 2 inches from plane 1, plane 1 would be allowed 67% of U (not
80%), and plane 2 would be allowed 33% (not 20%), because the allocation ratio cannot be more than 2 to 1.
When specified (see 6.2.g(4)), the residual unbalance for equipment with rigid rotors shall not result in vibration displacements larger than specified in Figure 2, when tested as specified in 5.2.3.2.
5.2.3 Balance procedure for flexible rotors.
5.2.3.1 Balance limits for flexible rotors. The residual unbalance for flexible rotors shall not result in vibration displacements larger than specified in Figure 2 when tested as specified in 5.2.3.2.
5.2.3.2 Vibration test procedure. When mounted as specified in 5.2.3.2.1 and measured in accordance with
5.2.3.2.2, the vibration displacement amplitude at the rotational frequency shall not exceed the values shown on
Figure 2.
5.2.3.2.1 Mounting. The unit shall be completely assembled and mounted elastically at a natural frequency corresponding to less then one-quarter of the frequency associated with the minimal operational speed of the unit.
To accomplish this, the minimum static deflection of the mounting should be determined by Figure 3, but in no case shall the deflection exceed one-half the original height of the elastic element. On machinery that cannot be mounted
as described, the unit shall be mounted on the shipboard mounting for which it is intended, as specified (see
6.2.g(5)).
5.2.3.2.2 Measurements. Amplitudes of vibration shall be measured on the bearing housing in the direction of maximum amplitude. On constant speed units, measurements shall be made at the operating speed. In the case of variable speed units, measurements shall be made at maximum speed and at all critical speeds (see 3.5) within the operating range. Measurements at many speeds may be required to establish the existence of critical speeds of variable speed units. The maximum frequency step size used when establishing critical speeds shall be 0.25 Hz.
5.2.3.2.3 Instruments. Amplitude and frequency measurements shall be performed with instrumentation which has calibration traceable to the National Institute of Standards and Technology (NIST), and which has dynamic and frequency ranges consistent with the amplitude and frequency range specified in Figure 2.
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